Aliphatic & Aromatic Hydrocarbon Analysis

46 Pages·2015·4.29 MB·English

Scientific Analysis Laboratories Ltd
An Introduction to Aliphatic and
Aromatic TPH analysis by two
dimensional GC / FID
David Smith, Technical Director
Scientific Analysis Laboratories Ltd
[email protected]
Scientific Analysis Laboratories Ltd
Aliphatic and Aromatic TPH – A New Dimension !
● A requirement of environmental fate and risk analysis is the separation of aliphatic
from the aromatic hydrocarbons
● Further breakdown to carbon banding chain lengths
The volatile fraction (C -C ) is analysed by GC/MS (headspace), where the aromatics
●
5 10
can be identified and quantified individually and hence separated from the aliphatics
using Mass Spectrometry and selected ions
● For years, the separation of aliphatic and aromatic fractions in the extractable fraction
(C -C ) has involved the use of a physical bench top separation technique such as Solid
10 40
Phase Extraction (SPE)
TPH CWG
Measuring the total concentration of petroleum hydrocarbons (TPH) in
●
soil does not give a useful basis for the evaluation of the potential risks
to man and the environment
Hydrocarbon fractions divided into aliphatic and aromatic fractions
●
and supplemented by analysis of single compounds. This will be a much
more useful basis for carrying out risk assessments
Criteria Working Group
The "Total Petroleum Hydrocarbon Criteria Working Group
(TPHCWG)" was formed in the USA in 1993 with the goal
to develop scientifically defensible information for establishing soil
clean up levels that are protective of human health at hydrocarbon
contaminated sites. The group had more than 400 participants from the
oil industry, consultant, several state governments and the US EPA. The
group has published 5 reports encompassing their findings and their
recommendations:
Vol-1- Analysis of Petroleum hydrocarbons in Environmental media
Vol-2- Composition of Petroleum mixtures
Vol-3- Selection of TPH fractions
Vol-4- Fraction specific classes and reference concentrations
Vol-5- Risk evaluation
Risk based Approach
Two sites may have TPH measurements of 500 ppm but constituents at one site
may include carcinogenic compounds while these compounds may be absent at the
other site

The risk at a specific site will change with time as contaminants evaporate,
dissolve, biodegrade, and become sequestered
A valid correlation between TPH and risk would have to be site- and time-specific,
related to a single spill, and, even then, the correlation might not be the same
around the periphery of a plume where the rate of compositional change accelerates
CWG Fractions
The American Petroleum Institute (API) that found it necessary to
modify the method slightly to incorporate evaluation of heavier
petroleum fractions. This resulted in a suggestion to prolong the
highest fraction (from EC35 to EC44 ), where aromatics and
aliphatics are assessed separately, and add an even heavier
fraction (EC44+ ) that includes both aliphatics and aromatics
Aliphatic fractions Aromatic fractions
>5 to 6 >5 to 7
>6 to 8 >7 to 8
>8 to 10 >8 to 10
>10 to 12 >10 to 12
>12 to 16 >12 to 16
>16 to 21 >16 to 21
>21 to 35 >21 to 35
>35 to 44 >35 to 44
CWG Fractions
Equivalent Carbon Numbers
The Equivalent Carbon Number, EC, is related to the boiling point of a chemical
.
normalized to the boiling point of the n-alkanes or its retention time in a boiling
point gas chromatographic (GC) column
Hexane contains six carbons and has a boiling point of 69o C. Its equivalent carbon
number is six. Benzene, also containing six carbons, has a boiling point of 80o C. Based
on benzene’s boiling point and its retention time in a boiling point GC column, benzene’s
equivalent carbon number is 6.5
This approach is consistent with methods routinely used in the petroleum industry for
separating complex mixtures and is standard for CWG analysis
Note that for molecules with higher relative carbon number indices, the disparity between
aliphatic and aromatic hydrocarbons is great (see next two slides)
Equivalent Carbon Numbers
Equivalent Carbon Numbers